Method for making a quasi-incompressible phase-change material with low thermal conductivity, and resulting product

ABSTRACT

A method for making a quasi-incompressible phase-change material (PCM) having low thermal conductivity includes combining with a phase-change material (PCM) in liquid state a thickening agent selected to reduce significantly thermal convection, the formed material having, depending on the combinations performed, a gelled structure or a colloidal dispersed system. The PCM may be a mixture of chemical compounds of the family of alkanes, paraffins, waxes, fatty alcohols, fatty acids and the like, and the thickening agent may be organic (aromatic ureas), organometallic (alkaline or alkaline earth soaps) or purely inorganic (silica, silico-aluminates such as bentonite made oleophilic). The material is useful for thermal isolation of containers or pipes, and in particular for thermal insulation of hydrocarbon carrying pipes.

FIELD OF THE INVENTION

The present invention relates to a process for producing aquasi-incompressible and low thermal conductivity material based onphase-change materials (PCM), products obtained with the process andapplications.

The material according to the invention can be used as a thermalinsulant in many spheres, notably for thermal insulation of pipescarrying fluids likely to undergo great changes of state under theinfluence of temperature: paraffin crystallization, hydrate deposition,ice formation, etc.

This is for example the case in the sphere of hydrocarbon production.Thermal insulation of subsea pipes notably turns out to be necessary inmany cases to keep the fluids flowing and to prevent as long as possiblethe formation of hydrates or of paraffin-rich or asphaltene-richdeposits. The development of deep-sea oil fields often combines thesedrawbacks which are particularly tough in case of production stops.

BACKGROUND OF THE INVENTION

Various thermal insulation techniques are described for example in thefollowing documents: FR-98/16,791, JP-2,176,299, or WP-97/47,174.

Thermal insulation can be obtained with various processes. Cellular orwoolly porous solid materials that block the convection of low thermalconductivity gas are used onshore or in shallow water. Thecompressibility of these porous materials does not allow this techniqueto be implemented at relatively great depths.

Another well-known technique consists in coating the pipe with a firstlayer of a paraffin-imbibed porous material for example, whose thermalinsulation coefficient is lower than that obtained with the gas trappingtechnique reminded above, and with a second layer of a refractorymaterial that reinforces the effect of the first layer. However, such asolution cannot be used in water.

Other solutions are more suitable for use at great depths of immersion.The following materials can be used for example:

-   -   coatings made of quasi-incompressible massive polymeric        materials based on polyurethane, polyethylene, polypropylene,        etc., which however exhibit a rather average thermal        conductivity, insufficient to prevent drawbacks in case of        production stops, or    -   coatings made of syntactic materials consisting of hollow balls        containing a gas and withstanding the outside pressure, embedded        in binders such as concrete, an epoxy resin, etc., whose        conductivity is lower than that of compact materials, but which        are much more expensive.

The pipe carrying the fluids can also be protected by means of anexternal pipe withstanding the hydrostatic pressure. A low thermalconductivity heat insulator left at atmospheric pressure or placed undervacuum, with partitions at regular intervals for safety reasons, is forexample interposed in the annular space between the pipes.

It is also well-known to interpose, between the pipe and a deformableprotective covering, an absorbent matrix sheathing the pipe, impregnatedwith a quasi-incompressible liquid/solid phase-change material at afusion temperature above that of the surrounding medium and lower thanthat of the fluids circulating in the pipe.

Phase-change materials (PCM) behave like heat accumulators. Theyreversibly release this energy during their solidification(crystallization) or absorb this energy during fusion. These materialscan therefore allow to increase the duration of production stops withoutpipe clogging risks through premature cooling of the pipe content. Knownexamples of phase-change materials are the chemical compouns of theC_(n)H_(2n+2) alkane series such as, for example, paraffins (C₁₂ toC₆₀), which offer a good compromise between the thermal andthermodynamic properties (fusion temperature, latent heat of fusion,thermal conductivity, heat-capacity rate) and the cost. These compoundsare thermally stable in the working temperature range considered, andthey are compatible with a use in a marine environment on account oftheir water insolubility and of their very low toxicity level. They aretherefore well suited for thermal insulation of deep-sea pipes forexample.

The state change temperature of these phase-change materials is linkedwith the number of carbon atoms of the hydrocarbon chain and it cantherefore be adapted for a particular application. In order to obtain aphase change around 30° C., a mixture of mainly C₁₈ paraffins can forexample be used, such as Limpar 18-20 marketed by the CONDEA AugustaS.p.A. company.

It is also possible to consider using waxes, normal paraffins, veryweakly branched (1 or 2 branches) long chained isoparaffins (C₃₀–C₄₀),long chained branched alkylcycloalkanes or long chained branchedalkylaromatics, also weakly branched, fatty alcohols or fatty acids.

Above their fusion temperature Tf, phase-change materials (PCM) are inthe liquid state and their viscosity is low. In order to overcome thisdrawback, which can be particularly disadvantageous in certainapplications, notably for the manufacture of double-walled vessels or ofenergy storage drums, it is well-known to add a thickening agent such assilica to solidify them and to prevent leaks.

Another drawback of phase-change materials (PCM) is that their viscousliquid state favours convection heat losses.

SUMMARY OF THE INVENTION

The process according to the invention allows to produce a material or aproduct based on quasi-incompressible phase-change materials (PCM)having a low thermal conductivity at a temperature above their fusiontemperature Tf.

It comprises combining with a phase-change material a thickening agentselected to greatly reduce the thermal convection at a temperature abovethe fusion temperature of the phase-change material.

According to an embodiment, the process comprises using a thickeningagent dispersed in the phase-change material.

According to another embodiment, the process comprises using athickening agent forming a gelled structure with the phase-changematerial.

The product based on low thermal conductivity phase-change materials(PCM) according to the invention comprises in combination a thickeningagent selected to greatly reduce the thermal convection at a temperatureabove the fusion temperature of the phase-change material.

According to an embodiment, the product comprises in combination aphase-change material (PCM) and at least one metallic soap, thiscombination being obtained by action of bases on fatty acids or fattymatter.

According to another embodiment, the product comprises in combination aphase-change material (PCM) and complex aluminium, calcium or lithiumsoaps obtained by in-situ neutralization of dissymmetrical acids.

According to another embodiment, the product comprises in combination aphase-change material (PCM) and at least one inorganic thickener(graphite, hydrophobic silica gel, silico-aluminates renderedoleophilic, etc.).

According to another embodiment, the product comprises in combination atleast one organic or organo-metallic thickener of aromatic polyurea typeor coloured pigments, dispersed in a phase-change material (PCM).

The product can possibly include antioxidant or antibacterial agents,corrosion inhibitors or an insoluble-filler intended to adjust thedensity or the thermal conductivity thereof, additives intended toimprove the stability thereof or a solvent intended to control theviscosity.

The product according to the invention finds applications for thermalinsulation in general. It can be applied in particular for thermalinsulation of hydrocarbon transport pipes, where it is used as a directcoating or interposed (injected) between the pipes and an externalprotective covering.

Other features and advantages of the process and of the materialproduced according to the invention, as well as application examples,are described hereafter.

DETAILED DESCRIPTION

As mentioned above, the process consists in dispersing, in aphase-change material (PCM), an insoluble thickening or gelling agentselected to reduce both the viscosity of the PCM and the thermalconvection of the PCM in the liquid state, so as to form ablocked-convection insulating substance having a half-fluid to solidconsistency.

The liquid component forming the continuous phase can be a mixture ofchemical compounds of the C_(n)H_(2n+2) alkane series such as, forexample, (C₁₂ to C₆₀) paraffins or waxes, normal paraffins, very weaklybranched (1 or 2 branches) long chained isoparaffins (C₃₀–C₄₀), longchained branched alkylcycloalkanes or long chained branchedalkylaromatics, fatty alcohols or fatty acids. The liquid componentpreferably represents 70% to 99.5% of the mass of the product.

The thickening agent forming the disperse solid phase can be of organicnature (aromatic ureas), organometallic (alkaline or alkaline-earthsoaps) or purely inorganic (silica, silico-aluminates (bentonite)rendered oleophilic by grafting an organic chain preferably comprising12 to 24 carbon atoms).

The thickeners generally come in the form of fibres, crystals, orlamellar or spherical particles, with very variable dimensions accordingto their chemical nature and to their preparation mode.

According to the nature of the thickeners, a composition of gelled ordisperse structure is obtained.

In the case of a gelled structure, the elementary particles of thethickening agent form, within the product, a coherent three-dimensionalnetwork (entangled fibres), with formation of internal bond strengths.The liquid phase-change material (PCM) is kept in the network bycapillary action.

In the case of a disperse structure, the elementary particles of thethickening agent are suspended in the PCM. The dispersion is ofcolloidal type. The stability of the thickener suspension depends on thedimensions and on the density of the particles, on the viscosity of themedium and above all on the inter-particle forces that allow the systemto be kept in equilibrium.

The efficiency of a blocked-convection phase-change material (BC-PCM)thus depends on four main parameters: the thickener concentration, theelementary dimensions of the thickener, the solvent power of the PCMtowards the thickener and the dispersion forces. A wise combination ofthese parameters allows to optimize the insulating power of the BC-PCMat temperatures above the fusion temperature Tf of the PCM. Variouscombinations are also possible.

EXAMPLES OF COMPOSITIONS ACCORDING TO THE NATURE OF THE THICKENINGAGENTS

1—The blocked-convection PCM can be based on metallic soaps: lithiumsoaps, calcium soaps, sodium soaps, aluminium soaps, or mixedlithium/calcium or calcium/sodium soaps. They are obtained in thepresence of liquid PCM, either by neutralization of fatty acids, or bysaponification of fatty matter by the following bases: lime, lithiumhydroxide, soda or aluminium hydroxide for example.

2—The blocked-convection PCM can also be based on complex aluminium,calcium or lithium soaps, obtained by in-situ neutralization ofdissymmetrical acids in the presence of liquid PCM.

3—The blocked-convection PCM can also be formed without soap, from:

3a—inorganic thickeners such as graphite or carbon black, a hydrophobicsilica gel or oleophilic silico-aluminates (montmorillonite, bentonite,etc.);

3b—organic or organo-metallic thickeners such as sodium terephthalate oraromatic polyureas or coloured pigments (indanthrene, copperphthalocyanine).

These compositions obtained without soap are formed by dispersion ofinorganic or organic compounds in the liquid PCM. These compounds areinsoluble in the liquid phase (PCM) at any temperature.

Additives

The following compounds can also be added to the compositions forcertain applications in order to provide certain specific properties.

1—Soluble Additives

a) Antioxidant additives can be added essentially when the product(blocked-convection PCM) undergoes a temperature rise during operation.The most commonly encountered additives are phenol derivatives(dibutylparacresol, etc.), sulfur-containing phenol derivatives andaromatic amines (phenyl • or • naphtylamine or alkyl amine diphenyls).These antioxidants retard the oxidation process through their inhibitingaction towards free radical formation or their destructive actiontowards the hydroperoxides formed;

b) antibacterial agents;

c) corrosion inhibitors;

c1) soluble in the liquid PCM, consisting of chemical compounds of polarnature, which are readily adsorbed on the metallic surface by forming ahydrophobic film (fatty amines, alkaline-earth sulfonate, etc.);

c2) water soluble and acting by passivation of the water phase (sodiumnitrite for example);

d) structure-modifying additives of polar nature (water, acetone,glycerol, etc.), intended to stiffen the structure of the entangled soapfibres or of the thickener and to improve the stability of thedispersion of the gelling agent in the PCM.

2—Fillers

Insoluble fillers such as hollow glass microballs, fly ash, macroballs,hollow fibres, etc., can be added to the BC-PCM in order to adjust itsdensity and/or its thermal conductivity.

3—Solvents

In order to fluidity the blocked-convection PCM, it is possible to usehydrocarbons of petroleum origin such as hydrocarbon-containingsolvents, distillation cuts, predominantly aromatic, naphthenic orparaffinic oils obtained through solvent extraction processes or deephydrotreating processes, solvents or cuts obtained by means ofhydroisomerization of paraffinic extracts of petroleum origin orFischer-Tropsch synthesis, solvents and compounds obtained by synthesissuch as, for example, ester type oxygenated compounds, synthesishydrocarbons such as hydrogenated polyolefins, etc.

The blocked-convection PCM thus consists of a combination of 70 to 99.5%by mass of liquid PCM and of 0.5 to 30% thickener, to which additives(<10%), fillers (5 to 60%) and solvents (0.2 to 5%) are possibly added.

Formulation Examples

The following product, consisting of 90% PCM, 9.5% lithium soap and 0.5%antioxidant, can be used as a blocked-convection PCM. Anothercomposition can comprise for example 90% oil, 2.5% dispersant (water,acetone, polar products), 7% oleophilic bentone and 0.5% antioxidant.

Applications

The blocked-convection PCM described above can be used for example forthermal insulation of subsea pipes.

The aforementioned patent application FR-98/16,791 describes a devicefor thermal insulation of subsea pipes intended to be laid on the bottomat a great depth. The device comprises an external coating consisting ofa quasi-incompressible liquid/solid phase-change material (PCM) havingan intermediate fusion temperature between the temperature of theeffluents circulating in the pipe(s) and the temperature of the outsideenvironment, and an absorbent matrix surrounding the pipe(s) as closelyas possible. The pipes and their coating are placed in a resistant anddeformable protective covering.

The external coating consisting of the PCM-impregnated matrix describedin the prior document can be advantageously replaced here by one of theblocked-convection PCM described above, which results in an improvementof the thermal insulation of the pipes and a simplification of theoperations of setting around the pipe(s), by pumping for example at atemperature above the fusion temperature Tf, very appreciable when theassembly of pipes to be insulated is complex.

Applications of the material for thermal insulation of pipes carryingfluids, notably hydrocarbons, have been described. It is however clearthat such a material can also be used for any other application where itis desired to have a very low thermal conductivity combined with anenergy release.

1. A process for producing a thermal insulant material based onphase-change materials (PCM), comprising combining a thickening agentwith a phase-change material comprising a mixture of chemical compoundsof the alkane series, the thickening agent being selected to reducethermal convection at a temperature above a fusion temperature of thephase change material, and forming, with the phase-change material, agelled structure.
 2. A material based on phase-change materials (PCM),comprising in combination, a phase-change material (PCM), and athickening agent comprising at least one organic or organo-metallicthickener or an aromatic polyurea or coloured pigments, dispersed in thephase-change material (PCM) and selected to reduce thermal convection ata temperature above a fusion temperature of the phase-change material,and at least one insoluble filler intended to adjust density or thethermal conductivity thereof.
 3. A material based on phase-changematerials (PCM) comprising in combination, a phase-change material (PCM)and a thickening agent selected to reduce thermal convection at atemperature above a fusion temperature of the phase-change material, thethickening agent comprising at least one metallic soap, and the at leastone metallic soap being obtained in the phase-change material by actionof bases on fatty acids or fatty matter, and further comprising at leastone soluble additive acting as an antioxidant or antibacterial additiveor a corrosion inhibitor or a substance modifying a structure thereof.4. A process for thermal insulation of pipes carrying fluids, comprisingcoating the pipes with a material comprising, in combination, aphase-change material (PCM), comprising a mixture of chemical compoundsof the alkane series, and a thickening agent selected to reduce thermalconvection at a temperature above a fusion temperature of thephase-change material, and forming, with the phase change material, agelled structure.
 5. The process as claimed in claim 4, comprisingcoating the pipes by interposing the material between the pipes and anexternal protective covering.
 6. The process as claimed in claim 4,comprising injecting the material into a space between the pipes and anexternal protective covering.
 7. The process as claimed in claim 6,wherein the pipes are deep-sea subsea pipes carrying hydrocarbons.
 8. Amaterial based on phase-change materials (PCM) comprising, incombination a phase-change material (PCM) and a thickening agentselected to greatly reduce thermal convection at a temperature above afusion temperature of the phase-change material, the thickening agentcomprising complex aluminium, calcium or lithium soaps obtained byin-situ neutralization of dissymmetrical acids, and further comprisingat least one soluble additive acting as an antioxidant or antibacterialadditive or a corrosion inhibitor or a substance modifying a structurethereof.
 9. A material based on phase-change materials (PCM) comprising,in combination, a phase-change material (PCM), at least one organic ororgano-metallic thickener or an aromatic polyurea or coloured pigments,dispersed in the phase-change material (PCM) a thickening agent selectedto reduce thermal convection at a temperature above a fusion temperatureof the phase-change material, and at least one solvent intended tocontrol viscosity.